In reference to NL Hold 'em cash games, it really depends on your natural talent, and whether you are playing online or live.

Online, you will run into problems trying to go over 10/20 NL. The 10/20 NL on party is fairly easy, but 25/50 NL on Ultimate Bet is way way above it difficulty-wise and most people will never be able to make that level.

Live, you run into a problem at 10/20 NL. The major 10/20 NL game I know of is at the Bellagio and is full of pro's.. not exactly easily beaten! I think assuming you get past that point, the 20/40 or 25/50 NL scene isnt that much tougher than the Bellagio game. However the 50/100 NL scene from sidegames from major tourneys is tougher yet. Above that and you run into the likes of Phil Ivey, Doyle Brunson, and Jennifer Harmon!